I would suggest filing an issue on the plugin's repository (I'm assuming you're using this plugin: GitHub - urbanairship/phonegap-ua-push: Urban Airship integration with Phonegap)
Not all plugins work with PGB; it's possible this plugin is one of them.
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I notice that UrbanAirship pushed a new version of their plugin recently which upgrades several dependencies (see diff). Some of these aren't yet supported on PhoneGap Build. Lock your UA plugin to 5.2.0, and all should work as before:
<plugin name="urbanairship-cordova" version="5.2.0" />